A common technology for extracting pure hydrogen from industrial streams, such as for hydrogenation for changing the balance of hydrogen in those streams or to increase reaction selectivity, is to use membranes of palladium or palladium alloys alone or supported structurally by a matrix. Membranes which contain thick enough palladium layers to be made without holes and not break during service tend to be expensive and have relatively high resistance to hydrogen permeation.
Several membrane variations and module designs have been proposed to minimize this effect. Membranes can include porous ceramics either by themselves or coated with palladium alloys or with silica and palladium coated refractory metals and alloys, especially those based on Nb, V, Ta, Ti, Zr. These have greater strength than palladium and palladium-based alloys, are cheaper per unit volume, and most have greater intrinsic permeabilities to hydrogen. Although the alternatives are less expensive than Pd, they are not less expensive compared to polymers. Thus, with all of these membranes more attention must be directed to module designs that make efficient use of the membrane surface and provide a high recovery percentage without undue gas-phase mass transfer resistance. To date, no commercial module has been described that is particularly efficient for large scale hydrogen extraction using any of these membranes.
An example of an apparatus for hydrogen separation is disclosed in U.S. Pat. No. 4,468,235 to Hill (Hill '235). The Hill '235 patent discloses an apparatus for separating hydrogen from fluids and includes, mounted axially in a cylindrical pressure vessel, a plurality of membranes in the form of tubes coated on either the inside or the outside or both sides with coatings having a high permeability to hydrogen. There is also a fluid flow inlet and a raffinate flow outlet and a header to collect hydrogen. No sizes or criticalities are disclosed for the extraction membrane. Additionally, since this design provides no mechanism for flow distribution or turbulence generation, the separation efficiency of this apparatus is not maximized.
Another example of a similar apparatus for hydrogen extraction is disclosed in U.S. Pat. No. 5,205,841 to Vaiman (Vaiman '841) issued Apr. 27, 1993. The Vaiman '841 patent discloses an apparatus for separating hydrogen from gas and gas liquid mixtures at low temperature. The Vaiman '841 apparatus includes a plurality of axially mounted tubes coated on both their inside and outside surfaces with palladium/platinum black. There is also a fluid flow inlet and a raffinate flow outlet and a header to collect hydrogen. Vailman '841 does not teach any sizes or criticalities for the extraction membrane or its arrangement within the structure. Additionally, as similarly stated above regarding the Hill patent, the Vaiman '841 design provides no mechanism for flow distribution or turbulence generation. Separation efficiency of this apparatus is not maximized.
Another typical design for large hydrogen extractors uses tubular membranes of palladium-silver alloy in spiral form. This tubing generally has an outer diameter of 0.0625 to 0.125 inches and wall thickness of approximately 0.003 inches. For the smaller diameter tubes, the source hydrogen flows over the outside of several wound helixs made from 10 to 15 feet of tubing. These, hydrogen extractors typically require complex expensive construction that limits heat and mass transport. Also, since pressure drops become excessive when the tube length exceeds about twenty-five feet large modules end up with 40 or more nested and stacked helixes that must be hand assembled in a large tubular bundle without damaging any single one of the delicate tubes. This is a delicate construction process by any standard.
Large diameter tubes avoid maldistribution and assembly problems by driving all of the flow through a single tube. The practical limit is reached at about 100 feet. Longer lengths lead to destructive harmonic vibrations, especially during start-up and shut-down. Also, since module size increases with the square of the tube diameter, such units have had to be too big to cite comfortably. Further, temperature uniformity is even harder to maintain than with 1/16 inch units.
The spiral type designs are particularly difficult to form when dealing with coated refractory metals or with ceramics, as these materials are more brittle than palladium and coated membranes require more gentle handling than homogenous palladium alloys. The spiral type designs inherently have problems with scraping of the membrane surfaces and with kinking of the tubing material during manufacture thereby leading to inherent weaknesses in the tubes which are utilized under pressure. For large scale applications, these spiral-type hydrogen extractors tend to be larger in overall size than the module of the present invention thereby adding to the cost of the structure, sitting, shipping, maintenance, manufacture, and making them unpleasant to the eye.
To date, modules based on tubular ceramics or ceramic-based membranes known are based on a single pair of concentric tubes. The diameter of the ceramic membranes is approximately 0.375". Such designs cannot be readily scaled up for commercial applications.
The present invention provides a hydrogen extraction module which eliminates the spiral-type extraction membranes and is much more simple to construct, more compact, and can be more easily constructed from difficult materials, such as ceramics, and from high diameter to wall ratio metal tubes.
The present invention also provides improved hydrogen recovery from relatively impure mixtures through the use of critically sized extraction membranes and turbulence generating bumps or packing.
Another approach to the problems of palladium based membranes recognizes that the specific alloys are chosen by a trade-off between cycling stability, ease of drawing, high permeance, lower volumetric cost, and relatively good surface properties. Currently, the single material that most closely meets all of these criteria are made from palladium-silver alloys containing 23 to 25% silver. These tubes typically do trade off moderate cycling stability. They typically do not break for about two years in operation and have moderate drawability against their relatively high expensive and high resistance to hydrogen permeation, especially at temperatures below 300.degree. C. and for gas streams containing sulfur, carbon monoxide, and olefins.
Several options to palladium-silver membranes have been suggested, but are not in common use. For example, the U.S. Pat. No. 1,292,025 to Darling discloses a membrane requiring porous or discontinuous palladium coat over a base of refractory metal Nb, V, or Ta. The U.S. Pat. No. 4,496,373 to Bohr et al. discloses alloying the palladium layer with silver, calcium or yttrium. The patent also requires an intermediate melt layer. The U.S. Pat. No. 4,536,196 to Harris discloses essentially a palladium membrane which is coated with various metals as poisons to prevent the fouling of the palladium surface. Under some circumstances, this poisoning can be advantageous to the surface properties of the membrane, but the high cost and low reliability of palladium remains. The U.S. Pat. No. 4,313,013 to Harris shows similar palladium membranes that have been in use.
The U.S. Pat. No. 3,350,846 to Makrides et al. discloses a process of purification of hydrogen by diffusion through a very thin membrane of palladium coated Group V-B metal.
The U.S. Pat. No. 5,215,729 ('729) issued Jun. 1, 1993, to the inventor of the present application and incorporated herein by reference teaches membranes which combine the strength and high permeation of refractory metals with a coating of palladium or palladium alloys to improve the surface properties of the membranes. As with single-layer palladium alloys, selectivity is essentially 100% for hydrogen extracted. Applicant has observed that some of the best refractory metals can be difficult to fabricate into tubes or modules. Applicant has further observed that the surface properties of some of these membranes was often far better than those of single layer palladium-silver, especially at low temperatures and in the presence of carbon monoxide, hydrogen sulfide, and olefins..
In view of the above, a further object of the present invention is to improve on the properties of palladium-silver and similar alloys by adding a coating of palladium or similar materials to improve the surface properties. The resulting membranes have good strength, ease of fabrication, good durability, relatively, low resistance to hydrogen even at low temperatures, improved resistance to carbon monoxide, H.sub.2 S and olefins, fair resistance to embrittlement, and a hydrogen selectivity that can exceed that for palladium-silver because the operating temperature is lower.